1. Field of the Invention
The present invention relates to a hybrid optical integration platform capable of incorporating an optical device or optical sub-module used for optical communication and optical signal processing in addition to optical waveguides and electrical wiring, an optical sub-module which can be equipped on an opto-electronic board, and a hybrid optical integrated circuit equipped with the optical device or optical sub-module, and to a process for fabricating the hybrid optical integration platform.
2. Description of the Prior Art
With recent advances in optical communication and optical information processing, development of an opto-electronic integration circuit is in demand. In such a circuit, active devices are incorporated in low-loss optical waveguides and the like to be driven by a high-frequency electrical circuit.
To achieve a circuit incorporating active devices on the optical waveguide and driven at a high-frequency, three conditions are required for the opto-electronic board. These are (1) a low-loss optical waveguide function, (2) an optical bench function to incorporate an optical device on the same substrate and prevent axis deviation, and (3) a high-frequency electrical wiring function required to drive the optical device.
However, a circuit that satisfies the above three conditions has not been obtained with the prior art.
As a prior art example, FIG. 1 is a schematic perspective view showing a construction called a "Silicon optical bench" in which, using a guide groove 2 and positioning reference surfaces 3a, 3b, and 3c formed on a silicon substrate 1, an optical fiber 4 and a semiconductor laser (LD) 5 are integrated on the silicon Substrate. In this construction, since the guide groove can be formed with a good precision utilizing good processability of the silicon substrate, integration of the optical fiber 4 with optical devices such as the semiconductor laser 5 and a photo-detector (PD) can be easily achieved. Further, since the silicon substrate is superior in thermal conductivity, it also functions as a good heat sink for optical devices.
Further, the electrical wiring 6 is formed directly on the surface of the silicon substrate 1, or through a very thin oxide film of less than 0.5 .mu.m in thickness, but this structure has a problem of considerably deteriorating the high-frequency characteristics of the electrical wiring 6. That is, to form the electrical wiring 6 with superior high-frequency characteristics, the electrical wiring layer must have a sufficient thickness and be formed on an insulator with small dielectric loss. However, the silicon substrate 1 is very thin, the resistance is not high enough to ensure high-frequency characteristics, and it has a specific resistivity of about 1 k-ohm.cm.
FIG. 2 shows the high-frequency characteristics of a 0.6 mm long coplanar wiring formed directly on the silicon substrate (T. Suzuki et al.: Microwave Workshop Digest (1993) p95). The axis of ordinates indicates the transmission characteristics S.sub.21 of the S parameter and the axis of abscissas indicates the frequency (GHz). Loss of the 0.6 mm long wiring is about 0.4 dB (2 GHz) and about 0.8 dB (10 GHz), which are converted to 1 cm as 7 dB (2 GHz) and 13 dB (10 GHz), thus showing a substantial loss.
On the other hand, in an optical packaged circuit having an optical waveguide function, the application of a silica-based optical waveguide formed on the silicon substrate is expected. Prior art optical waveguides include (1) a "ridge type optical waveguide" in which the core is protected with a thin over-cladding layer as shown in FIGS. 3A and 3B, and (2) an "embedded optical waveguide" in which the core is embedded in a sufficiently thick over-cladding layer as shown in FIGS. 3C and 3D.
FIG. 4 is a schematic perspective view showing an example of the ridge type optical waveguide which is described in a document. This document is "Hybrid-Integrated 4.times.4 Optical Gate Matrix Switch Using Silica-Based Optical Waveguides and LD Array Chips", IEEE J. Lightwave Technol., vol. 10, pp. 383-390, 1992, by Y. Yamada et al. This example shows a hybrid optical integrated circuit including a silica-based optical waveguide 7 formed on the silicon substrate 1 and a semiconductor optical device 8. In this example, a semiconductor laser amplifier (SLA) is represented. The optical waveguide 7 has a structure of a ridge type optical waveguide, in which a core 7a formed on a thick under-cladding layer 7c formed on the silicon substrate 1 is protected with very thin cladding layers 7b and 7c. The SLA 8 is surface packaged in an upside-down construction in which an active layer 8a is facing down in the vicinity of the waveguide end, and a heat sink 9 for heat dissipation is provided on the backside. Since, in such a structure, the core is only covered with very thin cladding layers 7b and 7c, it has problems that (1) the optical waveguide has a large loss, (2) it is liable to be affected by an external disturbance, and (3) formation of a directional coupler circuit is difficult. In particular, the directional coupler is an indispensable circuit element to fabricate a high-performance optical circuit, and the impossibility of its formation means that application of the ridge type optical waveguide is limited to a narrow area. Thus, the ridge type optical waveguide does not sufficiently satisfy the optical waveguide function. Further, the electrical wiring function is not investigated here.
FIG. 5 shows an example of an "optical waveguide circuit with terrace" (Yamada, Kawachi, Kobayashi: Japanese Patent Application Laying-open 63-131104 "Hybrid Optical Integrated Circuit") in which an optical waveguide is formed in a recess 1a on a silicon substrate 1 having irregularities, and a device is equipped on a protruded part 1b. In FIG. 5, an under-cladding layer 10c of a silica-based optical waveguide 10 is formed in the recess 1a of the silicon substrate 1, and a core layer 10b is formed on top, and finally an embedding cladding layer 10a is formed. The upper surface of the under-cladding layer 10c and the upper surface of the protruded part 1b of the silicon substrate are in line in height, and the protruded part 1b can be used as a height reference surface of the optical device 8. In such a substrate 1, the low-loss optical waveguide function and optical bench function are satisfied, but a function to provide high-frequency wiring is not considered at all. When electrical wiring is provided, it is formed on the protruded part 1b of the silicon substrate 1, which does not satisfy the requirements for high-frequency characteristics. In FIG. 5, the reference numeral 8a indicates an active layer, and 11 indicates a reference surface for device positioning.
FIG. 6 is a schematic perspective view showing the construction of a hybrid optical integrated circuit disclosed in Japanese Patent Application Laying-open No. 62-242362. This circuit almost comprises a buffer layer 12 provided on a silicon substrate 1, a silica-based optical waveguide 13 provided thereon, a device holding table 14 having the same height from the upper surface of the silicon substrate 1 as the buffer layer 12, a semiconductor laser 15 held in upside-down construction on the holding table 14, and an electrical wiring table 16 having a conductive film 16a electrically connected with a gold wire W to the upper electrode of the semiconductor laser 15 and protrudingly provided on the upper surface of the silicon substrate 1. The reference numeral 17 indicates a heat sink.
In such a circuit construction, since a difference in height from the upper surface of the buffer layer 12 to the core of the waveguide 13 is set equal to the difference in height from the upper surface of the device holding table 14 to the active layer 15a of the semiconductor laser 15, it has an advantage that optical devices such as a semiconductor laser can be equipped with a very high positioning precision.
However, even with this circuit, the optical waveguide 13 is limited to the ridge type, tends to be affected by an external disturbance, and cannot provide a low loss optical waveguide function.
FIG. 7 is a schematic perspective view showing construction of a hybrid optical integrated circuit disclosed in Japanese Patent Application Publication 5-3748. This circuit mainly comprises an optical waveguide 18 protrudingly disposed with nearly the same height on the silicon substrate, an optical fiber guide 19, an optical device guide 20, an electrical wiring holding table 21, a first conductive film (common electrode) 22 disposed on the silicon substrate 1, a second conductive film 23 disposed on the upper surface of the electrical wiring holding table 21 and insulated from the first conductive film 22, an optical fiber 24 disposed along the optical fiber guide 19, and a laser diode 25 as an optical device disposed along the optical device guide 20.
The circuit of this construction, since the optical device is equipped directly on the silicon substrate, has an advantage that the silicon substrate can function as a heat sink.
However, even with this circuit, the optical waveguide 18 is limited to the ridge type, tends to be affected by an external disturbance, and cannot provide a low loss optical waveguide function.
FIG. 8 is a schematic cross sectional view showing construction of an optical waveguide device disclosed in Japanese Patent Application Laying-open 5-60952. This device mainly comprises a silicon substrate 1, an optical waveguide 26 formed on the substrate 1, and a semiconductor device 27 equipped in an upside-down construction in a recess of the silicon substrate 1.
In the device of this construction, the optical waveguide 26 is formed on a convex region of the silicon substrate 1, and therefore an under-clad of a sufficient thickness cannot be formed. Therefore, it has a large transmission loss, tends to be affected by an external disturbance, and does not satisfy a sufficient optical waveguide function.
Further, in the above device, since the electrical wiring 28 is provided on the silicon substrate 1, the requirements for high-frequency characteristics are not satisfied.
As described above, the prior art hybrid optical integration technology does not satisfy the above three requirements. In particular, the high-frequency electrical wiring function has not been considered.